1. Field of the Invention
The present invention relates to a sheet feeding apparatus for reading image of sheet-like document by feeding from a document tray (a sheet storing unit) a series of sheets, each on which image of document is formed, to read the image of document formed on the sheet, and also relates to an image reading apparatus and an image forming apparatus, each equipped with the sheet feeding apparatus.
2. Description of Related Art
It is commonly known that an up-to-date image forming apparatus or the like is equipped with a document reading apparatus where a series of sheets each having image of document formed thereon are set by being stacked and stored in a document tray are automatically transferred sheet by sheet from the document tray to an image reading apparatus for sequentially reading the image of document formed on the sheet. The image forming apparatus equipped with such a document reading apparatus is designed to read a document and form an image at high efficiency. It is also demanded to read an image from a document, convert the read result into an electronic data, and form an image of the document from the electronic data obtained as described above at higher speeds (See Japanese Patent Application Laid-Open No. H05-94063A (1993) and Japanese Patent Application Laid-Open No. H07-301962A (1995), etc).
So far for high-speed processing of the sheet-like documents, the number of sheet-like documents stored in the document tray in a conventional document reading apparatus is substantially 100 to 200. Also, as the document reading apparatus has been improved and advanced at its sheet feeding unit, the types of documents to be conveyed are increased.
It is essential in the document reading apparatus permitting a series of sheets on which image of documents are formed to be set (stored) in the document tray and fed out in a sequence for reading the image of the document with an image reading apparatus to take out (pick up) the uppermost one of the sheets stacked in the document tray before feeding to the image reading apparatus. For the purpose, the document tray is used of a lift type tray (elevation/lowering tray) having a sheet elevation/lowering function (in upward and downward directions) (See Japanese Patent Application Laid-Open No. H11-199065A (1999)). Such an elevation/lowering tray has a sheet detector (a document detector) for detecting whether or not the sheets are correctly set in the elevation/lowering tray particularly throughout the elevation/lowering range of the elevation/lowering tray. When so, it is also detected whether the sheets are placed at their correct position at the deepest end of the elevation/lowering tray.
Such sheet detectors are employed as contact type sensors at the upper side or the lower side in either a common fixed type tray or an elevation/lowering tray of the document tray. The sheet detector has a bar-like contact thereof provided to come into contact with the leading end of the sheets so as to detect whether the sheets are correctly set or not.
However, when the number of sheets set in the document tray becomes larger, the contact of the sensor has to be increased in the overall length as shown in FIG. 1 and FIG. 2. More particularly, when a sensor S1′ is provided at the upper side of a document tray 22, the contact S1a′ of the sensor S1′ has to be increased in the area of the movement, that is, it is simply required to increase the length of the contact S1a′. This may cause interruption between the contact S1a′ and a guide-in roller R1, between the contact S1a′ and a pair of separate rollers R2 and R2a, and between the contact S1a′ and the axis of each roller. Also, as shown in FIG. 2, when a sensor S1 is provided at the lower side of the document tray 22, the movement area of its contact S1a may relatively be small. However, when there is a curled sheet SR and the number of the sheets in the stack SB is decreased, the detection of the sheets in the state where document tray 22 elevates may be difficult thus resulting in fault detection. Moreover, when the sensor located at the upper side of the document tray, not shown, and the number of sheets becomes smaller, a curled sheet may hardly be detected.
On the other hand, an optical sensor is used for projecting light onto the sheets from the upper side to the lower side or from the lower side to the upper side to detect the sheets in the document tray. However, such an optical sensor used as the sheet detector requires a troublesome adjustment for detecting the sheets from one type to another. In addition, its detecting operation is hardly applicable to not only transparent OHP sheets but also semi-transparent sheets or transparent portion contained sheets.